1. Field of the Invention
The present invention relates to image forming apparatuses such as a copying machine.
2. Description of the Prior Art
In a copying machine or the like, a lot of photointerruptors have been used so as to detect information required for control, for example, the size of recording paper, the position of recording paper, and the presence or absence of recording paper.
FIG. 4 shows a circuit for inputting detection signals of photointerruptors provided for paper feeding decks in a copying machine. In this example, the copying machine is provided with four paper feeding decks, and each of the paper feeding decks is provided with eight photointerruptors 1 to 32. Consequently, the total number of the photointerruptors provided for the four paper feeding decks is 32.
The 32 photointerruptors 1 to 32 are divided into four groups for each paper feeding deck. Output terminals of the eight photointerruptors for each of the groups are connected to eight input terminals I1 to I8 of a control section 300 of the copying machine through a three-state buffer 41, 42, 43 or 44 provided for the group. The control section 300 comprises a CPU (Control Processing Unit), a ROM (Read Only Memory) for storing its program and the like, a RAM (Random Access Read Write Memory) storing necessary data, and the like. Selection signal input terminals E of the four three-state buffers 41 to 44 are respectively connected to four different selection signal output terminals S1 to S4 of the control section 300. Each of the photointerruptors 1 to 32 is statically driven.
The control section 300 divides the three-state buffers 41 to 44 for the respective groups in time and sequentially selects the three-state buffers by selection signals, thereby to read detection signals of the photointerruptors 1 to 32 with the groups divided in time for each group.
Such a circuit for inputting detection signals of photointerruptors is complicated and the cost thereof is high. Accordingly, the applicant of the present application develops a circuit for inputting detection signals of photointerruptors as shown in FIG. 1. In this circuit, output terminals of eight photointerruptors for each group are directly connected to eight input terminals I1 to I8 of a control section 100 of a copying machine without passing through three-state buffers. The control section 100 comprises a CPU, and a ROM for storing its program and the like, a RAM storing necessary data, and the like. Driving power is supplied to the photointerruptors for each group through a switching transistor 51, 52, 53 or 54 provided for the group. Bases of the four switching transistors 51 to 54 are respectively connected to four different selection signal output terminals S1 to S4 of the control section 100.
The control section 100 divides the four switching transistors 51 to 54 in time and sequentially turns the switching transistors on by selection signals, thereby to dynamically drive photointerruptors 1 to 32 in units of groups. Specifically, the control section 100 drives the photointerruptors 1 to 32 by groups, according to a particular time for each group, thereby to read detection signals of the photointerruptors 1 to 32 with the groups divided in time for each group.
The above described circuit for inputting detection signals of photointerruptors shown in FIG. 1 has the disadvantage in that the photointerruptors 1 to 32 are dynamically driven, thereby to make it impossible to check to see whether or not each of the photointerruptors 1 to 32 is normal by a tester.
An object of the present invention is to provide, in an image forming apparatus in which a plurality of photoelectric detectors are divided into a plurality of groups, and the photoelectric detectors are dynamically driven in units of groups, whereby detection signals of the photoelectric detectors are read with the groups divided in time for each group, the image forming apparatus capable of checking the photoelectric detectors.